The present invention relates to a device for providing a Voltage Source Convertor (VSC converter) for converting direct voltage to alternating voltage, and alternating voltage into direct voltage in a plant for transmitting electric power. The VSC converter is connected to an active alternating voltage network having at least two phases and has a direct voltage side providing a direct voltage across at least one capacitor upon connection of the converter to the alternating voltage network. An inductance is connected between the alternating voltage network and the converter. A first breaking means is provided in each connection line between each phase of the alternating voltage network and the converter for connecting the respective alternating voltage phase to the VSC converter. The device conducts current to charge the capacitor through a resistance.
The VSC converter may be connected to a direct voltage network for producing high voltage direct current (HVDC). A plant for transmitting electric power between such a direct voltage network and an alternating voltage network has recently become known through the disclosure "PWM and control of two and three-level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995. However, it is emphasized that the invention is not restricted to this application. The VSC converter could, for example, be a part of an SVC (Static Var Compensator), in which the direct voltage side of the converter is not connected to any direct voltage network.
The number of phases of said alternating voltage network, and therefore the number of phase legs of the VSC converter, may be arbitrary. The invention is directed to a device for connecting a VSC converter to an alternating voltage network of at least two phases, however, there are usually three phases. The three phases of the alternating voltage network require three phase legs of the VSC converter, and therefore a total of six current valves, in which the converter constitutes a six-pulse bridge.
In a state in which a converter is not connected to the alternating voltage network, and there is in principle no direct voltage across said capacitor. When the converter is connected to the alternating voltage network, the capacitor will, in such a connection of the alternating voltage network, be charged through the diodes in the bridge of the converter, since the semiconductor elements, which are turn-off type, preferably IGBTs connected in anti-parallel therewith, are blocked and not controllable in this state. Energy is stored in the inductance as a consequence of the charging current. The inductance may be constituted by a transformer or inductors when the converter does not have any transformer. This energy will charge the capacitor beyond peak-rectifying voltage when the charging current sinks to an over-voltage condition. As a consequence, these systems have been adapted to conduct the current charging the capacitor through a resistance, which so far has been accomplished at high voltages, e.g., within the range 100 kV to 500 kV. A first breaking means connects a resistor into the respective connection line (phase leg), between the converter and the respective phase of the alternating voltage network, so as to reduce the charging current and drastically reduce the energy oscillating between the inductor and the capacitor. The resistor is then disconnected, so that the current in the respective connection line is shunted past the resistor so as not to cause unnecessary losses. Resistors have been incorporated in all the first breaking means, since these close simultaneously, and it is not known which of the phases of the alternating voltage network leads in time and, therefore, which one will initially charge the capacitor. This solution to the problem of over-voltage conditions across the capacitor occurring upon connection of a VSC converter to an active alternating voltage network when providing the converter with voltage is comparatively costly, and normally not accessible for so-called medium voltage breakers, for voltages between 10 and 100 kV.